The ability to reproducibly plate void-free uniform indium of target thickness and smooth surface morphology on metal layers is challenging. Indium reduction occurs at potentials more negative than that of proton reduction, and significant hydrogen bubbling at the cathode causes increased surface roughness. Indium (1+) ions, stabilized due to the inert pair effect, formed in the process of indium deposition catalyze proton reduction and participate in disproportionation reactions to regenerate Indium (3+) ions. In the absence of a complexing agent, indium ions begin to precipitate from solutions above pH>3. Plating indium on metals such as nickel, tin, copper and gold is challenging because these metals are good catalysts for proton reduction and are more noble than indium, thus they can cause corrosion of indium in a galvanic interaction. Indium may also form undesired intermetallic compounds with these metals. Finally, indium chemistry and electrochemistry have not been well studied, thus interactions with compounds that may serve as additives are unknown.
In general, conventional indium electroplating baths have not been able to electroplate an indium deposit which is compatible with multiple under bump metals (UBM) such as nickel, copper, gold and tin. More importantly, conventional indium electroplating baths have not been able to electroplate indium with high coplanarity and high surface planarity on substrates which include nickel. Indium, however, is a highly desirable metal in numerous industries because of its unique physical properties. For example, it is sufficiently soft such that it readily deforms and fills in microstructures between two mating parts, has a low melting temperature (156° C.) and a high thermal conductivity (˜82 W/m° K), good electrical conductivity, good ability to alloy and form intermetallic compounds with other metals in a stack. It may be used as low temperature solder bump material, a desired process for 3D stack assembly to reduce damage on assembled chips by the thermal stress induced during reflow processing. Such properties enable indium for various uses in the electronics and related industries including in semiconductors and polycrystalline thin film solar cells.
Indium can also be used as thermal interface materials (TIMs). TIMs are critical to protect electronic devices such as integrated circuits (IC) and active semiconductor devices, for example, microprocessors, from exceeding their operational temperature limit. They enable bonding of the heat generating device (e.g. a silicon semiconductor) to a heat sink or a heat spreader (e.g. copper and aluminum components) without creating an excessive thermal barrier. The TIM may also be used in assembly of other components of the heat sink or the heat spreader stack that composes the overall thermal impedance path.
Several classes of materials are being used as TIMs, for example, thermal greases, thermal gels, adhesives, elastomers, thermal pads, and phase change materials. Although the foregoing TIMs have been adequate for many semiconductor devices, the increased performance of semiconductor devices has rendered such TIMs inadequate. Thermal conductivity of many current TIMs does not exceed 5 W/m° K and many are less than 1 W/m° K. However. TIMs that form thermal interfaces with effective thermal conductivities exceeding 15 W/m° K are presently needed.
Accordingly, indium is a highly desirable metal for electronic devices, and there is a need for an improved indium composition for electroplating indium metal, in particular, indium metal layers on metal substrates.